Photoresist is a light sensitive material used in certain fabrication processes to form a patterned coating on a work piece, e.g., a semiconductor wafer, during processing. After exposing the photoresist coated surface to a pattern of high energy radiation, a portion of the photoresist is removed to reveal the surface below, leaving the rest of the surface protected. Semiconductor processes such as etching, depositing, and ion implanting are performed on the uncovered surface and the remaining photoresist. After performing one or more semiconductor processes, the remaining photoresist is removed in a strip operation.
During ion implantation, dopant ions, e.g., ions of boron, boron difluoride, indium, gallium, thallium, phosphorous, arsenic, antimony, bismuth, or germanium, are accelerated toward a work piece target. The ions implant in exposed regions of the work piece as well as in the remaining photoresist surface. The process may form well regions (source/drain) and lightly doped drain (LDD) and doubled diffused drain (DDD) regions. The ion implant impregnates the resist with the implant species and depletes the surface of hydrogen. The outer layer or crust of the resist forms a carbonized layer that may be much denser than the underlying bulk resist layer. These two layers have different thermal expansion rates and react to stripping processes at different rates.
The difference between the outer layer and bulk layer is quite pronounced in post high-dose ion implant resist. In high-dose implantation, the ion dose may be greater than 1×1015 ions/cm2 and the energy may be from 10 Kev to greater than 100 keV. Traditional high dose implantation strip (HDIS) processes employ oxygen chemistries where monatomic oxygen plasma is formed away from the process chamber and then directed at the work piece surface. The reactive oxygen combines with the photoresist to form gaseous by-products which is removed with a vacuum pump. For HDIS, additional gases are needed to remove the implanted dopants with oxygen.
Primary HDIS considerations include strip rate, amount of residue, and film loss of the exposed and underlying film layer. Residues are commonly found on the substrate surface after HDIS and stripping. They may result from sputtering during the high-energy implant, incomplete removal of crust, and/or oxidation of implant atoms in the resist. After stripping, the surface should be residue free or substantially residue free to ensure high yield and eliminate the need for additional residue removal processing. Residues may be removed by overstripping, i.e., a continuation of the strip process past the point nominally required to remove all photoresist. Unfortunately, in conventional HDIS operations, overstripping sometimes removes some of the underlying functional device structure. At the device layer, even very little silicon loss from the transistor source/drain regions may adversely affect device performance and yield, especially for ultra shallow junction devices fabricated at the <32 nm design rule or below.
What is needed therefore are improved methods and apparatus for stripping photoresist and ion implant related residues, especially for HDIS, which minimizes silicon loss and leaves little or no residue while maintaining an acceptable strip rate.